Re: phase reversal: can an LM393 do this?
- From: "Tim Williams" <tmoranwms@xxxxxxxxxxx>
- Date: Sun, 19 Feb 2006 17:22:57 -0600
Incidentially, you didn't mention a pull-up for the LM393. It has an
open-collector output, not a driven output as most op-amps have, so you need
to source current for it to make any signal. It might be using the feedback
resistor as a supply resistor, giving even whackier behavior.
Anyway, you can't use an LM393, or 311, or 339 for that. A friend of mine
supposedly actually managed to stabilize one linearly, but all my efforts
have resulted in a relaxtion oscillator around 3MHz, no matter how much
capacitance I put on any pins.
The internal circuit is essentially identical to any other op-amp, although
most have more output bits, current sourcing and current limiting for
instance. The fundamental difference is the lack of exactly one capacitor,
that slows down the amplifier at high frequencies. Without compensation,
you get an amplifier that responds "too fast", overshooting where it wants
to be, so it compensates by undershooting -- again, too far, too fast. The
result is it "hunts" around the average level, but will never stabilize at
it.
You should get some NE5532's or something.. 741 is out of production
anyway(!).
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
"Anthony Fremont" <spam@xxxxxxxxxxxx> wrote in message
news:xe6Kf.18384$UN2.14455@xxxxxxxxxxxxxxxxxxxxxxx
I was tinkering with a circuit that I put together about a year ago. It
listens to the tick tock of a clock using an 8 ohm transistor radio type
speaker as a microphone. This circuit uses a single 5V supply. I have
an LM741 with the + input biased at half of Vcc (2.5V... yeah I know
already it's a sucky op-amp for that, but it's what I had on hand). The
speaker is connected to both inputs differentially thru 3.3uF caps.
Gain is set to 1000 using a 100K feedback and 100R input resistor on
the - input. The output is fed thru a 47K resistor to an LM393
comparator's + input. The minus input is biased to about 1/2 Vcc but
just a wee bit smaller than the value coming from the 741 so that its
output idles high when no sound is being heard. The comparators output
is positively fed back thru a .33uF cap and series 100K resistor. This
makes the comparators output snap low and stay there for about 120mS
after the sharp attack of the tick (or tock as appropriate) decays below
the idle offset voltage between the comparators inputs. This all works
well and fine, but.
Looking at the scope, with loud ticks, a mS or three after the output of
the comparator snaps low, it instantly snaps high for a few mS and then
back low again. The PIC chip trying to measure the spacing between
ticks and tocks, doesn't like this. It seems that this happens when the
output of the 741 exceeds a certain point. Apparently the 393 doesn't
like its inputs coming too close to Vcc? Does this sound likely? The
reason I suspect this is that I was able to cure the problem by placing
some cool blue LEDs on the output of the 741 (before the 47K resistor).
One LED between the output and Vcc, and one between the output and
ground. The LEDs have a typical Vf of 3.3V, but both light dimly with
the 2.5V applied. When the ticks (or tocks) occur, the LEDs flash
brightly while keeping the output of the 741 between ~1.8V and ~3.3V
above ground. No more funny "phase reversals" on the output of the 393
after that. The 393 didn't seem to care how near the voltage went to
ground.
Sorry so long, but I want to be clear. :-)
.
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